Antennas can be an afterthought in many designs. With the deployment of 5G, the need to get the most out of the antenna will only become more important.
Josh Mickolio, Digi-Key Electronics
There are a lot of assumptions, confusion and questions about how 5G will impact the non-handset world. There are several high-level goals for 5G. One is to offer technology that can better support faster speeds with enhanced Mobile Broadband (eMBB). Another is a continued offering of the LPWA technologies like Cat-M and NB-IoT with massive Machine Type Communications (mMTC). A third is to provide a better service level for critical infrastructure applications with Ultra-Reliable Low-Latency Communications (URLLC). Any one of these would be a big leap forward. The reality is that 5G will mean different things to different users.
Consider the first specification from the 3GPP organization (3gpp.org) that will utilize frequencies in the millimeter-wave (mmWave) spectrum. The bandwidth available at these frequencies—a few gigahertz– is massive compared to that available at the lower bands used with 4G LTE, tens of megahertz per band typically. The use of mmWave spectrum dominated 5G discussion from the start due to the impressive performance gains that have been demonstrated relating to network speed.
Millimeter-wave is only a small part of the 5G story. Though millimeter-wave may dominate the discussion, the sub-6-GHz bands are where most of the devices deployed will reside. The term for the mmWave bands is FR2 (frequency range 2). The sub-6-GHz bands are known as FR1. Within FR1 will be the existing 4G LTE spectrum as well as a couple more areas known as C-Band and CBRS, where the frequencies are around 3.5 to 4 GHz.
Antennas will be quite physically different across these ranges of spectrum used by 5G. First, the full wavelength of the mmWave signal is below 1 cm, whereas at 600 MHz the full wavelength is almost a half a meter. Thus there is a vast difference in antenna sizes. Antennas are typically quarter-wave or half-wave in length; at mmWaves the antenna can be tiny given that a half-wave antenna is 5-mm long at 30 GHz. A half-wave antenna at 2.4 GHz is a little over 6 cm long or 12 times bigger.
Another consideration concerns the signal properties and particularly the signal propagation properties. At mmWave frequencies, the signal is easily blocked or absorbed by obstacles like construction materials and water though lower frequencies can pass through these materials more easily. A key number in any wireless link is the link budget, which is a sum of transmitted and received power gains and losses in the system. The link budget can be a useful tool in estimating usable range at a given frequency.
The general equation for the link budget is just Power Received = Power Transmitted + Gains – Losses. The units are typically in dB. Typical gains and losses include antenna gain (Tx and Rx), receiver sensitivity, path loss, cable/connector losses, and noise figure.
Path loss is how much the signal diminishes or attenuates over a given distance. It can be affected by the environment, reflections of the signal and many other factors. Path loss is often the largest loss in the link budget. It rises significantly as the frequency increases. For example, the free-space path loss at 600 MHz is 68 dB over 100 m; at 30 GHz it is 102 dB.
It’s useful to consider the impact of antenna dimensions on planned 5G and existing 4G installations. The 5G NR FR1 (New Radio, Frequency Range 1) utilizes many of the same frequencies as 4G. Fortunately, this new band won’t affect existing 4G equipment. 5G and 4G will operate in a dynamic spectrum-sharing model, meaning the same antenna will work for both 5G and 4G in those bands. This spectrum-sharing model also may mean that the life cycle of a 4G LTE device should be extended more than previous generations like 3G.
The performance of 4G and 5G in the lower bands in FR1, at least for now, are not significantly different. More good news is that the cellular LPWA (Low-Power Wide-Area) technologies–or Cat-M and NB-IoT which were developed several years ago to operate on existing 4G LTE bands—meet the requirements for mMTC in 5G. This means that these devices can already be considered 5G devices, and again no changes are needed to the antenna.
One factor receiving a lot of attention is the new bandwidth available for operator use in the millimeter-wave (or close to it) spectrum. These bands (the most common bands are n257, n258 and n261) are around 24 GHz up to 40 GHz. They offer a major potential throughput advantage over the other bands and will work well for certain use cases. In the real world, however, high path losses at these frequencies complicate the deployment of mmWave 5G across a wide area. Consequently, the limitations of mmWave make it impractical for most applications that must span long distances.
The antennas used at lower frequencies (FR1) are fairly simple, often taking the form of a simple length of metal. The radiation pattern is somewhat isotropic or close to it in the intended directions. These antennas are also likely passive, with no active components to boost the signal or filter out noise.
In contrast, wwWave antennas are often more complex. To boost the range to usable levels at mmWave frequencies we must improve the link budget. There are a few common approaches available: increase the transmit power, increase the antenna gain or directivity on the transmit and receive signal path, or lower the path losses.
There is a limit to how much power can be transmitted, especially in smaller devices the size of a mobile phone. Path losses are difficult to reduce when you have no control over the path between the transmitter and receiver and thus can’t remove obstacles, raise the device off the ground for better line of site conditions, and so forth. An antenna with a higher gain can increase the range, and highly directional (beam-forming) steerable antennas are frequently used to get the most out of a wireless link at these frequencies.
A phased-array antenna can concentrate signals in a specific direction to enhance gain. The array consists of a matrix of patch antenna elements each fed the same signal that is delayed by fixed amounts via a phase shifter. By adjusting the phase of the signal as it goes to each element, constructive and destructive interference is used to steer the radiation pattern, essentially aiming the beam where desired.
At mmWave frequencies phased-array antenna elements are much smaller than those operating at lower frequencies, making larger matrices feasible. The more antenna elements used, the narrower and more directional the RF energy. Still, it’s difficult to fit phased-array antennas into most embedded applications due to size, cost and complexity, even at mmWave frequencies. There may be future use cases that can utilize lower-cost antennas, but it is unlikely that mmWaves will make an impact in most embedded applications anytime soon.
There is another approach to 5G that can be viewed as a compromise between the advantages of mmWave bandwidth and the longer ranges available at the lower frequencies. It involves the new spectrum available to network operators, C-band and CBRS. C-Band has recently been turned on with some U.S. network operators, but the 5G band n78 has been used in Europe and Asia for a while. The band n77 will be used in the U.S. The importance of C-band when it comes to 5G can’t be overstated. It provides a significant increase in available bandwidth, anywhere from 40 to 200 MHz, with more to come. It also sits at a frequency that is low enough to get usable range.
These bands will likely not be utilized for low-power IoT devices and won’t impact this area. But we do see devices like gateways and routers becoming available that take advantage of the added bandwidth. Antennas in use today, especially multi-in-one external antennas, may already support C-Band. One metric for antenna performance is the efficiency at any given frequency. Some antennas will operate at higher efficiency at certain bands and lower efficiency at others. To get the best performance, it’s important to pay attention to the bands that will be used most and the antenna efficiency in those bands.